Studies of the transmural distriubtion of myocardial blood flow will be performed in chronically instrumented awake dogs using 7-10 micron diameter radionuclides-labeled microspheres injected into the left atrium. A study of the effect of cardiac contraction on myocardial strains during cardiac contraction by comparing transmural mycardial blood flow in the right and left ventricles when arterial inflow is limited to the interval of systole. Since shearing forces are similar in the right and left ventricles while transmural tissue pressure during systole is markedly different in the two ventricles, it will be possible to dissociate the effects of these two variables on transmural myocardial blood flow. In a second study, regional myocardial blood flow will be measured while coronary blood flow is restricted by a varible proximal coronary artery stenosis and distal coronary artery pressure is measured. Coronary vasomotor paralysis will be produced by administration of adenosine. The resultant data will define the relationship between coronary perfusion pressure and subendocardial perfusion in the absence of coronary autoregulation. The effect of exercise on myocardial blood flow subsequent to a myocardial infarction will be examined in the chronically instrumented awake dog. This study will evaluate the ability of myocardial blood flow distal to a totally occluded coronary artery to increase in response to exercise.